Long ago, in a far-off land, there was a moment in time where a bit of pooled magic briefly touched the spirit of a recently deceased dragon. A bright flash of light ensued, and upon fading, there it was: a Guardian Spirit, born of flame, stone, and a touch of magic.
A few days of confusion ensued, wherein the newly-formed being named itself, and latched onto a small cadre of dwarven engineers. They trekked west together, the dwarves teaching the ghostly apparition their language and values as they settled into sheer bluffs between a coastline and a wild jungle full of the bones of ancient creatures. Calling their new home An Bhalla, they constructed a grand wall and began the decades-long task of turning the bluffs they had settled into an imposing fortress.
Unfortunately, this task was never completed, for in the second decade of work upon the Bulwark, a hard truth was spoken through the mouth of a quetzalcoatl: the world was doomed.
Little can be said about the end of a world that is new or interesting. But in this case, there are some points of interest. Firstly, an empire far to the south and east of An Bhalla had engaged in some sort of magical experimentation that released a corrupting Blight, centered upon the riven remains of the empire, and quickly spreading to all corners of the world.
Secondly, there was the possibility of escape for all inhabitants. The quetzalcoatl who had revealed the doom of the world could also protect its inhabitants from the spreading Blight, if they could make their way to the network of caverns in the furthest northwest reaches of the world. Pirates, undead wizards, rough men from northern tundras. bird-men, dwarves - all could be saved.
With a remarkable effort, all the once-fractious nations constructed their arks, some - like the dwarves of An Bhalla - constructing stasis chambers powered by the reaction of strange ores and others simply settling in to wait for the time to rebuild.
When the world ended, all was thrown into a void of silence. There was little talk, and less food. Years passed, and things grew quieter, until only a small core of folk were left. They worked together, repurposing the machines and buildings constructed to take them through the end of a world, and created a machine to take them to other worlds.
Other worlds that would prove to be lively, and a place for the remaining few to find food as well as meaning, during the interminable wait for their own world to become fit for habitation. But one of their number could not use the machine as he was: the Guardian Spirit. The machine needed a framework more solid than ectoplasm, bone, and ancient armor to work upon. So, the spirit invested the last of his own powers, and formed himself a more solid body from the bones of the earth. Then could he join the rest of the group, now calling themselves Thunderbears, in raiding other worlds for sustenance.
And the Thunderbears arrived upon many worlds, with a rumbling bass and a bright light. Some worlds were bright and full of life, while some were nearly lifeless. Some inhabitants had the power of gods and ancient sorcerers behind them to drive off the interdimensional travelers, while others could not resist and fell to their knees before the leader of the small band: the Wizard Sheez and his right-hand badger, named Fonjo.
The hardened warriors grew in skill, both combative and artistic, and their mandate become to not only claim portions of unknown worlds for themselves, but to demonstrate their craft to the inhabitants in ways that could not be mistaken for mere luck.
The Guardian Spirit, now a hot-tempered dwarven crossbowman, was often the scout for these worlds, and would sometimes keep to himself in peaceful places where he could refine his art. He remains a skilled long-range combatant and battleground engineer, despite a tendency toward harsh words and rash actions.